Can't open .BLEND files
You could check Windows registry using: regedit and manipulate these entries:
In case none of the entries are found in the registry you could save them in a text file e.g. "blend.reg" and import the settings by regedit.

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\blendfile]
@="Blender File"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\blendfile\DefaultIcon]
@="\"C:\\Program Files\\Blender Foundation\\Blender\\blender.exe\", 1"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\blendfile\shell]

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\blendfile\shell\open]

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\blendfile\shell\open\command]
@="\"C:\\Program Files\\Blender Foundation\\Blender\\blender.exe\" \"%1\""